Chlorophyll as a benchmark for vegetable quality

Without blanching, the enzymes catalase and peroxidase will accelerate changes in texture, color, and flavor even after the food has been frozen. There are several ways to follow changes in vegetables after processing, including measuring ascorbic acid or, in the case of cauliflower, the brown pigments soluble in a water-acetone mixture. Many T-TT studies, however, followed the fate of chlorophyll.

The chlorophyll content of a living plant is readily measured. The blanching procedure ends the respiration process, and the conversion of chlorophyll to pheophytin begins. At the same time, the color slowly changes from a bright green to a dull olive brown. With proper handling and processing, it can be assumed that at the beginning, when the food is freshly frozen, there is no pheophytin present, and any changes in the ratio of chlorophyll to pheophytin are taking place in the post-freezing chain of events from processor to consumer. This proved to be a sensitive technique for the WRRC to follow what was happening as time and temperature fluctuated. It was actually possible to construct empirical equations that would predict the chlorophyll conversion based on the temperature history of a frozen vegetable.

The T-TT investigators found that "the first perceptible loss of initial quality of many frozen vegetables occurs in less than one year’s storage at 0° F." Further, "a few degrees difference in temperature for long-term storage can cause a great difference in quality retention. At the same time, short exposures above 0° F are no more damaging than long exposures at lower temperatures." (5) These results testify to the complexity of the frozen-food stability problem. For example, on storage at 0° F, the first noticeable flavor difference occurred first for peas (265 days), followed by spinach (271 days), green beans (285 days), and cauliflower (355 days).

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Early methods of food preservation | A chance discovery | Frozen food chemistry | U.S. Agriculture turns to science | Frozen food research begins at WRRC | Defining "Quality"Chemical reactions at low temperaturesChlorophyll as a benchmark | Major scientific results from the T-TT programSocietal impact of the T-TT program | Landmark designation | Further reading and acknowledgments

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